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Journal of Coastal Research
785-S00
I
Fort Lauderdale, Florida
The Effect of Tide Range on Beach Morphodynamics
Morphology: A Conceptual Beach Model
Summer
1993
and
Gerhard Masselink and Andrew D. Short
Coastal Studies Unit
Department of Geography
University of Sydney
Sydney, New South Wales ~:OO6,Australia
ABSTRACT
MASSELINK,
G. and SHORT, A.D., 1993. The effect of tide range on beach morphodynamics and
morp.hology: A conceptual beach model Journal of COCI8talResearch, 9(3). 785-800. Fort Lauderdale
(Flonda), ISSN 0749-0208.
Natural beaches may be grouped into several beach types on the basis of breaker height (H ) wave period
('!'), hia:h tide sediment fall vel~ity (~,) and tide r8lll!e (TR). These four variables are qu:n'tified by two
andSHORT
(1984)
dlmens~onl~
par~eters:
the dimensionless fall velocity (0 - HJw,T) used byWRIGHT
to classify ml~ro-tld.al beaches, and ~e !elative tide range (RTR - TR/H..) introduced in this paper. The
or dissipative surf zone
valu~ ~f the ~Imenslo.nless fall ve~oclt~ mdicates whether reflective, intenaediate
condl.tlons will prevail. The relative tide range reflects the relative impodance of swash, surf ZODeand
shoalmg wave. processes.
A co~ceptual model i~ presented in which beach morphology (beach t.nJe) may be predicted using the
.
dimensionless fall velocity and the relative tide range, whereby the mean spring tide range (MSR) is used
to cal~ulate the re!Ative tide r8lll!e. The model consists of the existing miao-tidal
beach types, which as
RTR I~creases, shift from reflective to low tide terrace with and finally wi&hout rips; from intermediate
to low tide bar and ~~s an~ finall! ultr~-diasipative; and from barred dissipative to non-barred dissipative
and finally ultra-dissipative.
USing this model, all wave-dominated bead1esin
all tidal ranges can be
.
classifil'd.
ADDmONAL
change.
INDEX WORDS:
INTRC.DUCTION
On all natural bea.ches, processes and morphology arepredomiI1laDtly influenced by waves
and tide. Whereas the importance of waves is selfevident and well documented (WRIGHTet al., 1984,
1985), the influence (Jlftides, though recognised
(e.g. WRIGHTet al.j 1987), is more subtle and less
well understood. Tide ranges have been classified
by DAVIES(1964) as being mlcro- « 2 m), meso(2-4 m) or macro-tid:al (> 6 m). Consequently,
beaches can be classified accordingly. .However,
as DAVISand HAYES(1984) indicated, beach morphology is not simply dependent on the absolute
wave height or tide range, but on the interaction
of the two.
.
Existing micro-tidal beach models assume a micro-tide range « 2 D1)in the presence of oceanic
waves. Therefore, they can not automatically be
applied to macro-tidal beach environments. SHORT
(1991) addressed this problem, suggesting the micro-tide threshold be I"aisedto 3 m, and proposing
a grouping of macro-tidal beaches into higher wave
"
92017 received 2A:iFebruary 199)!; accepted in revuion
6 December 1992.
Beaches and tide range, micro-tidal,
maero-tidal,
beach model. beach
planar, moderate. wave multi '-bar, and low wave
to tidal flats. .This grouping, while illustrating the
range of morphologies associated with macro':tidal beaches, is still based largely on wave height
and does not enable differentiation based on tide
range> 3 m.
This paper addresses this problem by combining wave height and tide range into a singledimensionless parameter, and calibrating the applicability of this parameter using field
data and
~
the literature.
BACKGROUND
Several beach models are available to predict
beach state as a function of wave and sediment
parameters (SONU, 1973; WRIGHT and SHORT, 1984;
SUNAMURA,1989; LIPPMANN and HOLMAN, 1990).
The models are generally representative of microtidal beaches and do not take account of the tide.
Numerous studies (reviewed later) have also investigated the effect of tides and increasing tide
range 'on beach morphodynamics. However, the
overall contribution of tides to beach morphology
remains unresolved.
The model of WRIGHTand SHORT(1984) is use-
.,.~o,.;..III"u...U
'au
es with highl)
.able tide ranges, sediment sizes
ful hr describing the morphodynainic variability
and morphologies. In order to examine the relaof micro-tidal surf zones and beaches. This model
tive contribution of both Hb and TR, this paper
describes plan and profile configurations of six
combines and extends the ideas in WRIGHT and
major beach states. In addition to providing a
SHORT (1984) and SHORT (1991) by considering
spatial classification. the model enables~he pre~
the relative effects of waves and tides on beach
diction of beach change and equilibrium beach
morphology. Following a suggestion in DAVISand
states (WRIGHTet ai., 1984,1985). The beach states
HAYES(l984:Figure 8), the relative tide range RTR
are related to wave and sediment characteristics
is introduced as a new parameter. The relative
via the dimensionless fall velocity, 0 -= Hb/w,T
tide range is given by the ratio of tide range to
(GOURLAY.1968; DEAN. 1973), where- Hb is the
breaker height (RTR = TR/H,J. Large values of
breaker height (m), w. is the sediment fall velocity
RTR indicate tide-dominance
and small values
(m/see) and T is the wave period (see).
express wave-dominance.
According to WRIGHT and SHORT (1984), conA conceptual model is presented according to
ditions when 0 < 1 result in a reflective beach
which the beach state is a function of dimensionstate. The beach face will be steep and is generally
less fall velocity n and relative tide range RTR.
cusp ed, and a pronounced step is usually present
Beaches on the macro-tidal central Queensland
at the base of the swash zone. Generally wave
(Australia) coast and the literature are used to
height is small and beach sediments are relatively
illustrate the model.
coarse. Intermediate
beaches have values of 0
ranging from 1 to 6 and are characterized by bar
EFFEcr OF TIDES ON SURF
and rip morphology. Four different intermediate
ZONE DYNAMICS
beach states are defin~d and with increasing 0,
According
to
DAVIS (1985), tides playa passive
these states are low tide terrace (LIT), transverse
or indirect role in sediment transport and changes
bar and rip (TBR), rhythmic bar and beach (RBB)
in beach morphology. The primary role of the tide
and longshore bar trough (LBT). When n > 6,
is to alternately expose and submerge a large porthe beach is in a dissipative state. On dissipative
tion of the beach and the inner surf zone. The net
beaches. the wave energy level is generally high.
result of this movement of sea level is to retard
sediments are fine and the surf zone is wide. Subthe rate at which sediment transport and changes
dued bar morphology may be present but rips are
in morphology take place. This may be illustrated
usually absent.
by the findings of DAVISet ai. (1972) who showed
The model of WRIGHT and SHORT (1984) was
that bar migration rate decreases with increasing
developed on and for micro-tidal environments
tide range.
(TR <2 m) and the tide range is not accounted
However, the tide does more than just retard
for, preventing application of the model to envisurf zone processes. During a tidal cycle. the poronments with larger tide ranges. The first, and
sition of the swash zone, surf zone and shoaling
so far only. attempt to include the tide into some
.
wave zone is shifted with the tide both vertically
Bort of conceptual beach model is by SHORT (1991)
and horizontally. causing the intertidal beach prowho proposed a tentative grouping of micro- to
file to be influenced to varying degrees by each of
macro-tidal beach and tidal flat systems. Accordthese processes every 12 hours. This is illustrated
ing to SHORT (1991), beaches with macro-tidal
in the results of a simulation model developed by
, ranges (> 3 m) may form the transition between
MASSELINK(in press) which investigates the imwave-dominated
micro-tidal beaches and tideportance of swash, surf zone and shoaling wave
dominated tidal fiats. He distinguishes three types
processes over the beach profile as a fU!lction of
of macro-tidal beaches on the basis of the wave
relative tide range.
energy level. High waves (Hb >0.5 m), and parThis simulation model calculates over half a
ticularly swell. produce moderate gradient (1-3°),
tidal cycle (from low water to high water) the
concave, planar beaches. Moderate waves and sea
relative amount of time that different locations
conditions result in lower gradient (0.5°), multi bar
on the beach profile are in the swash, surl and
(ridge and runnel) topography. As wave energy
shoaling wave zone. The swash zone is defined as
drops even further, a third type is produced with
the zone between the quasi-stationary mean sea
a high tide beach fronted by a tidal flat.
level (tide level) and the maximum runup height.
This grouping, however, is primarily based on
The surf zone is defined as the area between the
and
as
a
result
includes
in
single
groups
beachH"
.
.J,,"m..1
"r
Conceptual
-'U.......
r.OIO~tJl' RPI'''''rch- Vol. 9. No- 3. \993
tide level and the.
in!! wave
lIebreak point and. the shoal-
zone extends
seaward
from
the
~
point. For each zone, an empirical relationship
between the equilibrium beach gradient and wave
and sediment characteristics is assumed. The simulation model combines these relationships with
the relative occurrences of swash. surf zone and
shoaling wave processes for different parts on the
beach profile and computes an "equilibrium"
beach profile, Input parameters to the model are
wave height, wave period, sediment size and tide
range. For more details. see MASSELINK(in press).
Figure 1 illustrates the results of the simulation
mode] using wave height = 1 m, wave period = 8
sec. sediment-fall velocity = 0.03 m/sec and varying the tide range from 2 to 15 m. It shows that
the contribution of swash and surl zone processes
decreases as the relative tide range and the contribution of shoaling waves increases. Swash and
surf zone processes always dominate the upper
intertidal and have a.see.ondary maximum around
low tide level where the- tide level remains relatively stationary during the turn of the tide. The
lower part of the intertidal profile, however, may
become completely dominated by shoaling waves
for large relative tide ranges (RTR > 10). Other
results of the simulation model further suggest
that as the contribution of shoaling waves increases with increasing tide, beach gradient decreases (MAssELINK, in press).
The dominating role of shoaling waves on
beaches with large relative tide ranges has been
verified in the field by WRIGHT et ai. (1982b) who
conducted field experiments
on Cable Beach.
Western Australia (mean spring tide range 9.5 m
-and RTR = 12). They concluded that under modal
wave conditions in the low- to mid-tidal zones,
most ol the work was performed by unbroken
shoaling waves rather than surf zone processes..
and only in the high tide zone did surf zone processes dominate.
The movement of the three morphodynamic
ZODes across the profile during each tidal cycle
has strong implications for cross-shore bar formation and morphology. Laboratory studies have
shown that the presence of a tidal range inhibits
offshore bar formation (e.g. WATTSand DEARDUFF.
1954), while HEDEGAARDet ai. (1991) found that
the formation of bars is suppressed when RTR >
3. From field experiments on micro-tide beaches,
WRIGHT et ai. (1986, 1987) concluded that increasing tide range resulted in more subdued bartrough topography during spring tides, compared
''''''n~.'
,..1'('
1D
Heach Model
787
!«iN rw
~IOOuvn
~g
~~
,
LOwI1Df
U'YfI.
75
~~
e!j so
g't:
.~;
2S.
~:;
.c
~
0-0
0.25
0.50
0.75
non-dimclUional
1.00
1.25
1.50
dcpch rclativc 10 hilh tide IeYeI
occurrence o( awash and aurl wne proceasn
beach profile (or relative tidr ranCeI (TRI
H.) 0(2. 3, 5, 10 and 15 ca1culaud over half a tidal cycle (low
tide to high. tide). The importance oC awash and surC wne processea deereuea as relative tide range increases. Swuh and lurC
wne proceaaes ahow a aecondary maximum oCoccurrence around
low tide level, where the tide remaina relatively Il4tionary Cor
BOrne time. The relative occurrencea are calculated by . limu.
lation model described by MASSELINK (in preaa). The input
Figure 1. Relative
- over
a dimenaionle..
parameten
to the
8 .ee, lediment
5, 10, 15 roo
model
-
are wave
Call velocity
0.03
height
m/aec
- 1m,.ave period
- 2,3,
and tide
ranee
to the more accentuated topography during neap
tides.
In addition, tides have three pronounced effects
on three dimensional water circulation in the
nearshore zone. First, in rip current systems...!iE!
are strongest at low tide. when the water is sufficiently shallow to concentrate the flow oC the
current within the rip channels (SHEPARDet 0/.,
1941). Maximum rip current velocities occur during the falling tide (McKENZIE. 1958); and according to COOK (1970), on most beaches, rips
become better developed when the tide is falling
or low. Second, offshore directed bottom flow (often misnamed as "undertow") is~enhanced
at
the falling tide, as shown by RUSSELLet 0/. (1991).
who monitored suspended sediment transport
during
a storm
(Hb
=3
m) on a beach with a large
tidal range (up to 9 m). They found a distinct
asymmetry in the cross-shore suspended sediment transport about high tide, with little net
transport on the flooding tide, and strong net offshore transport on the ebbing tide- Third, on
beaches with large tide ranges, shore-parallel tidal
currents play an increasing role in longshore sediment transport on the lower intertidal and subtidal zone of beaches (WRIGHT et al..-1982b).
The following points may bCtsummariz~ about
the influence of the tide on beacnmorphodynamics: (1) incr~_asing ti!i_e_J'8Ilg~ft!ta.rds the rate at .
., ~n.n...
L "...' n
""'-
1--
.
Masselink and Short
788
Conceptuel
Beach Model
789
T.Ye L Mean .spri",! tide range. infe~ modal wave conditions. sediment c""roderisti..
ro,.,e ond tM dimenslonleu faU velocity of the studied beaches.
.
Q
...
<10:'"
.
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a
4 ...~"'...";''" -!~ q
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"0
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-.
:
i
5
T
(see)
D..
(mm)
4.9 '\
4.6
3.6
U
4.6
3.6
~.~
4.9'
3.9
4.6
3.6
3.6
.4$
3.9
3.6
0.4
0.6
0.3
0.3
0.6
0.36
~.!=
0.6
0.76
0.4
0.4
0.6
0.3
0.76
0.6
5
5
6
6
6
6
~
6
6
6
6
6
6
6
6
0.87
0.69
0.29
0.34
0.52
0.24
c.we B.
9.6
0.8
and IiARnISTY(1984)
PtmcIine Sands
7.6
0.8
c..- 88y
Anastrong
/"
.
~ -
-...
-'...
"
B.
H8rbour B.
Eo.aPark B.
NiBe MileB.south
s.nioa B.
Niae MUeB.central
B.
~
Famborough
I"8mborough
Coral
Sea
B. south
B. central
"'.y
Mile B. north
-Fanborough B~lIorth
W-.rT
J-
RTR
0
0.30
0.26
0.18
0.16
0.21
0.13
0.21
0.14
12
8
12
16
8
10
6
10
6
12
9
6
16
6
7
0.6
1.3
1.3
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1.6
1.9
2.2
2.4
3.7
4.1
4.1
4.1
4.7
6.1
6.9
10
0.26
12
2.4
7
0.17
9
6.8
16
9
3
0.6
Queenslaud Field Silel
eo.- TreeB.
~bert.s B.
..p '"p
-,
H.
(m)
Ce-.I
...
. ...F',.
"I.. "'",
".
".
MSR
(m)
Lacation
N
~q
5
""'"
...
for tM m,h lide beoch. relotWe lide
u.oo
et aL (1982b)
KJM;(1972)
BI8ckpooI B.
Draridce Boy
MSR
- ~uediment
Ficure 2. The maao-tidal. centnl QueeD8landcoastline aDd the eo-tidal lines of the muimum spring. tide range (eo-tidallines
after BP A, 1979). Depth contours are in meten.
which sediment transport and' morphological
changes take place, (2) an increase in tide range
results in an increase in the importance of shoaling wave processes which in turn prodQce lower
beach gradients, (3) large relative tide rlmges inhibit offshore bar formation and even on microtidal beaches can suppress accentuated bar-trough
morphology (RBB and LBT states), (4) rip circulation and seabed return flow is enhanced at
low tide and diminishes at high tide, and (5) on
macro-tidal beaches shore parallel, usually reversing, tidal currents become increasing domi.
nant seaward of the lower intertidal zone.
CENTRAL
QUEENSLAND
MACRO-TIDAL
BEACHES.
In order to investigate the morphological
characteristics
of beaches with large (relative)
tide
ranges, several beaches on the macro-tidal
central
Queensland coast were selected for field investigations (Figure 2). The aims were to 8II!M!88
the.
relative contribution of grain size, wave height
and tide range to beach morphology. To achieve
this, a number of sites were selected having different sediment, wave and tide characteristics.
Following a visual survey of around thirty
beaches located around Mackay and Yeppoon
(Figure 2),11 were selected for more detailed field
investigation, with 15 transects surveyed acroai
these beaches. For the purpose of this study, the
following data were collected at each site: beach
morphology and cross-sectional profile, beach
sediments, modal wave height and period and tide.
characteristics.
All beaches were surveyed at low tide using a
theodolite and leveL Sediment samples were collected from the high tide swash zone and analyaed
using a settling tube. Wave characteristics were
extracted from the Queensland Beach Protection
7.6
0.61
6?
0.22
U .
0.61
61
0.84
.
spring tide range (m): H.-inferred IIIOdaIbreaker beicht (m); T -.modal wave period
size (1IUD):RTi\.-:-relati4eticierange (MSR/IIJ: 0 dimensionless fall velocity
-
Bumd's coastal observation prograni(COPE) and
filum offshore wave rider stations at Yeppoon
(SPA, 1979) and Mackay (BPA, 1986). Tidal
chracteristics
are published for the Standard
.-u
Hay Point and Mackay, and the secondary
(MR1s Port Clinton and Rosslyn Bay (Figure 2) in
Australian Tide Tables (1992). The high tide beach
&ecliment size; modal wave height and period are
used to
compute
the dimensionless
fall velocity
9, and the mean spring tide range together with
tile modal wave height is used for obtaining the
rela1ive tide range (Table 1). Cross-sections of all
listed beaches are illustrated in Figure 3. They
haw been positioned on the basis of their dimenaioDleas fall velocity (0) and relative tide range
(B.TR) value.
The beaches with a mean spring tide range <4
m are. from the Yeppoon region, the other beaches
are found around Mackay. In addition to the central Queenaland beaches, several beaches from
other macro-tidal coastlines, extracted from the
literature, are also listed in Table 1.
Based on Table 1 and Figure 3, four types of
beaches can be identified. The Queenalandbeach-
es .with 0 <2 and RTR
(see)'
. D..
meaD
<15 (Lambert's
hich
tide
Beach,
Harbour Beach, Emu Park Beach, Grass Tree
Beach, Cooee Bay) all possessed three character~
istica. Firat, a steep reflective high tide beach,
usually cuaped and composed of coarser sand.
Second, the high tide beach terminates at a distinct break in slope and sediment, and finally,
seaward of the slope break is a lower gradient,
finer sediment, more dissipative,low tide terrace.
Usually the beach groundwater outcrop (effiuent
line) is located at the slope break, resulting at low
tide in a dry upper beach and a wet low tide
terrace.
Where 0 > 2 and RTR < 15, the beaches either
have a low gradient mid-intertidal zone and barf
rip morphology around low tide level (Nine Mile
Beach north,. central and lOuth, F~borough
Beach central) or are fiat and featureless throughout (Famborough Beach north and lOuth, Sarina
Beach, Bucasia Beach). These two types of beachesean not be discriminated on the basis ofO. They
do, however, have distinctly different relative tide
ranges (Table 1, Figure 3). The barred beaches
all have relative tide ranges <7, while the fiat and
\.,,,,,
I
D IMENSI
12345
0
riJ.,~/ntlUJd
':
-4
L
- - '6.u",p~ip
0
Nin~~i~c B.
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~
~~
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L TT
.
Lamben's
B.
--.
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~.
.
JUu...
L TT
Faroborough
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~
P'
12
L~T_.
>
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LS
_ LTT
Gr.ISS Tree B.
Cooee Bay
.
::;E
OIblcB.
fow~~~ce-+
- - - - -
...,
rip-:-
...,
-;-
16
Blackpool
~V)
B.
Armstrong B.
barred
barred dissipative
(~
Mtp".,.,tIi
~
- KII
100
.[~~
"-
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-
.
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.
6,uplH.dif-
--
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~
...,
- - - - - - - - - -
-"-,
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'100
lOG
)GO
ultra-dissipative
'~,-,
~..
- -
1(1)
-
...,
.; ..., ...,
...,
...,
- - - - - - - - - - - - - - - - - ...,- ...,...,
- - - - - - - - -' - - - -
:
., ;
.w..a...J-.bipl;t......
-
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.
5
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.
.--,
..
DISSIPATIVE
0
- - '- - - - - - - - - - -
low tide terrace
8
~...4
.:
0
.~
BucasiaB.
'1!:11
11
or-
~/~nnd
fI41tUlJ
IUIlurdus
-
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Sarina B.
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d
Sands
-
Emu Park B.
-l
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.~ ~
~~_.
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f~ahuJC$S
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r
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P'
Drungcay
d B
f'.-du~
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Farnborough B. nonh
Harbour B.
~.
~
E-<
~
~
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Farn~~~;:'fh
L TT
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8
~
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)000004
--..
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reflective
::r::
.~.
ih'/, .Hul!t:J
INTERMEDIATE
.
.J::i
NinemileB. central NinemileB. nonh
G/~":,,,/..
200
REFLECTIVE
.0
~&'elnINUl
J"
"~~;P~M"PNSJ6N~LESSFAtLVEtObhIy~i~~Qi=-tHb/W~T
0
6
4
.D
::r::
............
~
I
I
T
ONLESS.FALt'Y-ELPCITY.."i(2.;::Hb/ws
'-I)lU".
-
.
.
~.tttlJull4nku
.~~1"~~*~;T"~
IOO,:IDO
...,
...,
_.- - - - - 1- - - - - - - - -'-
JOCI
...,
- - - - - - - - - - - - -
G
...,
- - -
transition to tide-dominated tidal flats
5GD
IiCIO
...,
...,
~
~
- - - - - - - - -
Figure 3. Plot of macro-tidal central Queensland beach profiles and several other macro-tidal beaches listed in Table 1. The beaches
have been positioned in the graph on the basis of their dimensionless fall velocity (0 = Hbw,T) and relative tide range (RTR = TR/
Hb), Mean spring tide range (MSR) is used to calculate RTR and the high tide sediment fall velocity is used to calculate O. Positioning
Fig.,. 4. Conceptual beach model. Beach slate is a function of dimensionless fall velocity (0
= HJw,T) and relative tide rang~
(RTR
TRIHb). HT and LT refer to mean high tide and mean low tide level, respectively.
of the beaches is based on the center of the beach profiles, but in order to avoid overlapping of beach profiles, the position of some
beaches may be slightly off center. The origin of the profiles is the mean high water spring level and the dashed lines indicate the
mean low water spring leveL Beach morphology is indicated on each profile including low tide terrace (LIT) and three dimensional
bar/rip topography. Note that examples of micro-tidal beaches where RTR <3 are not shown, nor are tidal flats where RTR
:> 15-
having some salt tolerant vegetation around the
high tide level.
featureless beaches have RTR >7. This suggests
that when {} > 2 a RTR threshold exists around
7, which controls the presence « 7) or absence
(> 7) of low tide bar and rip morphology.
Macro-tidal beaches with rhythmic topography
at low tide level are not uncommon on the central
Queensland coast. Analysis of three series of aerial
photographs from the area north of Yeppoon
showed numerous beaches, including Nine Mile
Beach, with various low tide bar configurations
and rhythmic wave lengths ranging from 150 to
300 m. The photos were taken just before the
tropical cyclone season, suggesting that the bars
reflect modal trade wind wave conditions, rather
CONCEPTUAL BEACH MODEL
Figure 4 presents a conceptual model based on
the micro-tidal beach literature (RTR <3) and
the macro-tidal beach literature and field data
illustrated it.. F!gt.!!'~3 ~d Tz.b!8 1 fv~ RTR > 3.
The usefulness of the dimensionless fall velocity
{} in classifying micro-tidal beach morphology is
well documented and in Figure 4 is indicated by
those beaches where RTR <3. Figure 3 shows that
as both n and RTR increase, a logical sequence
of change in beach morphodynamics occurs and
that micro- and macro-tidal beaches may be morphologically gr{)uped into different beach types
on the basis of these two dimensionless parameters.
Joumal
than the artifact of a high wave cyclone event.
Unforturmtely, the majority of these beaches are
not readily accessible and, heI!ce, exact morphodynamical data is not availab!e.
Finally, the Queensland beaches with large relative tide ranges (RTR > 15) are fronted by very
fine inter- and sub-tidal sediment (0.1 mm) and
a rippled very low gradient « 0.5°) intertidal zone.
The upper intertidal zone, however, may be either
steep and relatively coarse grained (Armstrong
Beach) or may consist of very fine sediments and
have a very low gradient (Ball Bay). These beaches form the transition to tidal flat environments
(SHORT'S (1991) Group
of Coastal Reaearch,
Vol. 9, No.3,
1993
3) as indicated
by Ball Bay
-
Journal
The different beach types illustrated in Figure
4 are: reflective beaches, grading with increasing
RTR into low tide terrace beaches with rips and
low tide terrace beaches without rips, intermediate barred beaches grading as RTR increase into
beaches with bar/rip-morphology at low tide level,
dissipative beaches with subdued bars produced
by low RTR, grading with increasing RTR into
featureless non-barred dissipative beaches, and
finally both low tide bar/rip and non-barred dissipative beaches grading into ultra.dissipative
beaches when RTR > 7. When RTR > 15, the
beaches begin the transition to tidal flats along
the lines of SHORT'S(1991) Group 3. Even larger
relative tide ranges (RTR ::>15) will result in true
,tidal flats.
In the following section, each beach type is discussed and illustrated with examples from the
of Coastal Research,
Vol. 9, No.3,
1993
\
Conceptual
793
Masselink and Short
792
central Queensland sites and from the coa~talli~erature. Unfortunately, many sites descn~ed In
the literature provide only limited informatlo~ on
characteristics such as wave height and period,
tide range, grain size and beach gradient (~rofiles). Therefore, many well known macro-tidal
beach sites cannot be used owing to the lack of
sufficient environmental documentation in the
literature.
IMPACT
Reflective
\
Beach Model
Reflective
OF SPATIAL VARIATION
0 AND RTR
Group
IN
(D < 2)
Beaches
Fully reflective beaches only exist in environments when 0 < 2 and RTR < 3. The beach face
is steep and commonly cusped, and a pronounced
coarse step is usually found at the base of the
swash zone fronted
bya
lower gradient,
finer
grained subtidal zone (WRIGHT and SHORT, 1984).
The height of the step increases with wave height
and grain size (HUGHES and COWELL, .1987;
SUNAMURA, 1989). Waves are generally surgmg or
plunging on the beach and most of the wave energy is at incident
and subhannonic
(twice the
Low Tide Terrace without Rips
As the relative tide range exceeds 7, the very
wide and dissipative low tide terrace becomes increasingly dominated by unbroken shoaling waves
(see Figure 1) which produce a uniform, featureless low tide terrace (e.g. Harbour Beach, Figure
5b) without the formation of rip channels. According to CARTER (1988), this type of low tide
terrace beach is commonplace on high latitude
coasts where the high tide beach consists of gravel
fronted by a fine grained low tide terrace. KOMAR
(lG,oJ shows an exampie oi this type oi beach
from the coast of Wales (Figure 11-6, p. 297) and
KING (1972) presents data on Druridge Bay
(Northumberland,
England), a low tide terrace
beach with ridges and runnels (Figure 3). In addition, they have been reported along the ?ulf of
California (INMANand FlLLOux, 1960) and m NW
Western Australia (HESP, personal communication).
wave period) frequencies
(HUNTLEY and BOWEN,
1975; WRIGHT and SHORT, 1984).
Low Tide Terrace with Rips
As relative tide range increases to be~wee~ 3
and 7, and 0 remains <2, the steep, reflective high
tide beach remains, while a relatively flat terrace
forms around low tide level with rips. Usually the
h' h tide beach consists of significantly coarser
se~iments than the low tide terrace. The textural
discontinuity is associated with a distinct break
of slope and often the low tide beach gr?und v:a~r
outcrop (effluent line) is located at thiS position
saturating the low tide terrace (Figure 5a). Beach
cUsps may be present around high tide level and
the low tide terrace can be dissected by small
(mini) rip channels.
. .
At high tide, surf zone processes are similar to
those on reflective beaches and waves are generally breaking or surging up the beach fac.e, ~he~eas during low tide the surf zone will be dlsslpat~ve
with several lines of spilling breakers. For relative
processes
tide range values of up to 7, surf ~one
lay a significant role on the low tide terrace re~ulting in the formation of rip channels.
Intermediate
Group
(D
Beaches
For the lowest relative tide ranges (RTR <3)
several types of bars may occur (e.g. LIPPMANN
and HOLMAN, 1990). The bar-topography
may
consist of alternating
transverse
bars and rips,
whereby the bars are attached
to the shoreline
and rip currents flow between the bars (e.g. SONU,
1972; WRIGHT and SHORT, 1984; SHAW, 1985; JAGGER et al., 1991). Alternatively,
the bar may have
a sinuous crescentic
form (e.g. GREENWOOD and
DAVIDSON-ARNOTT, 1979; GoLDSMITH et al., 1982;
MGAARD, 1988a) or be linear (SALLENGER et al.,
1985; WRIGHT et al., 1986). WRIGHT et al. (1986,
1987) found that even on micro-tidal
beaches increasing spring tide range suppresses
the formation of the LBT and RBB with their deeper shorelinear troughs in favour of shallower rip-driven
TBR. Thus, an increase in (relative) tide ranges
will result in more subdued bar-morphology,
but
with enhanced rip circulation
at low tide.
Low Tide BarJRip
As the relative tide range increases (RTR = 37), the beaches maintain the relatively steep up-
,
1
"
-
:
-
::
,
-.
'
:'.~,':',..,.,.~~~
,.~""~'~'~:--~'.,,,'.'_':".~:,.- "~..'~':~'El"""'~.'.~'.'
,\.,
':~
',,""
,.
",-
. "-',..,
--
-,'
,,'-,:~".'..
::---'-: :"'::"':'::'
= 2-5)
Interrqediate values of the dimensionless fall
velocity (0 = 2-5) and a relative tide range of <7
will result in beaches with distinct bar-morphology and cellular rip circulation.. Two types of
beaches may be distinguished.
Barred
.
.
Figure 5. Examples of the di1rerent beach types for relative tide ranges larger than 3: (a) Low tide terrace with rips. Embleton
Bay, Northumberland
(U.K.) is a beach eJ:pooed to the North Sea with a mean 8pring tide range of around 4.5 m and coane high
tide beach sand. The photo 8hows the beach with a dry, reflective and c:usped upper part of the profile, a wet and dissipative low
tide terrace, and rip currents In the 8ubtidal zone~ (b) Low tide terrace without rips. Harbour Beach (Mackay, central Queensland)
is a low tide terrace beach with a relative tide range of around 8 and a II value of 1.5. The upper part of the beach is steep and dry,
and
evidence of cusp\ng is visible at the lower end of the beach. A flat and wet low tide terrace characteriaea the lower part
of the Intertidal profile. Under modal conditions this beach is reflective at high tide and dissipative at low tide. (c) Low tide barf
rip. Nine Mile Beach (Yeppoon, central Queensland) is characterised by a relative tide range of 5-6 and II is around 3.5. Transverse
har-rip morphology is present on this beach 88 Indicated by the dye pattern. The upper part of the profile is cbaracterised by the
of a low but extenaive 8wash bar. During a spring tidal cycle, morphodynamic signatures on this type of beach are compleL
p"""""
At spring low tide, the 8urf is dissipative, from low-to-mid tide the transverse bar.rip 8ystem operates, from mid-to-high tide the
8urf zone is once again dissipative, and reflective conditions may occur at 8pring high tide. (d) Non-barred dissipative. Rhossili Bay,
South Wales (U.K.) is expoeed to Atlantic 8well, consists of line sediments and is 8Ubject to a mean 8pring tide range of 9.5 m. The
Cable
beeeh is wide, planar and feature1ess. DissipatiVe surf zone conditiona prevail throushout the tidal cycle. (e) mtra-dlssipative.
Be8dt (Broome, Weatem AustraIia) is an ultra-dissipative beach with a relative tide ranee of 12 and II is aroun4 2.5. Although the
Intennediate
value of II Indicates that rhythmic topography may develop, the relative tide range is too large, resulting in a flat and
featureless lower Inter- and suhtidal profile. A subdued ridge and runnel is often present around neap high tide level, while reflective
coaditiona can produce cusps at 8pring high tide. (0 mtra-dissipative.
Farnborough Beach south (Yeppoon, central Queensland) is
an uIUa-dissipative
beach with a relative tide range of 6 and II is around 4. The Inter- and suhtidal profile is featureless and dissipative
8urf zone conditiona prevail throughout the tidal cycle.
per iQtertidal zone, but are fronted by a low gradient mid-intertidal zone, possibly with swash
bars, and then bar and rip morphology around
low tide level (e.g. Nine Mile Beach, Figure 5c).
These beaches have more complex morphodynamie signatures and may experience reflective (high
tide), iQtermediate and dissipative (low tide) surf
zone conditions through the tidal cycle. The bar
and rip morphology only exists on the low tide
beach and is active only on either side of low tide.
Recent work on Nine Mile Beach suggests that
the bar and rip morphology is mainly driven by
processes occurring during neap (low) tides (unpublished
data). The higher energy central
Queensland beaches (Nine Mile Beach and Farnborough Beach central) and the meso-tidal beaches (TR
4 m; Hb
1-2 m) of the Oregon (AGUI-
""
'"
LAR-TuNON and KOMAR, 1978; Fox and DAVIS,
1978) and the New England coasts (ZEIGLER and
TuTTLE, 1961) belong to this group.
~.
"
--~-"
------
~-
794
..
.Dissipative Group (0 > 5)
Rip currents and associated rhythmic topography are generally absent on dissipative beaches
(WRIGHTet al., 1982a) and the surf zone is characterized by the presence of numerous spilling
lines of breakers. Sediments are often fine to very
fine and the beach gradient is low.
Barred
Cunceplual
Mas:selink and Short
Dissipative
Beaches
For !J > 5 IU)d RTR <3, the dissipative
beach
profile will be characterised
by subdued longshore
bar-trough
morphology.
Waves are of the spilling
type and the water motion in the inner surf zone
will be dominated
mass transport is
a strong offshore
the return current
GREENWOOD and
by infragravity
waves. Onshore
by spilling waves and bores while
directed bottom flow dominates
pattern (WRIGHT et al., 1982a;
OSBORNE, 1990).
Non-Barred Dissipative Beaches
As RTR increases >3, the dissipative beaches,
while maintaining similar dimensions, become
flatter and more featureless with no bars. Examples are Farnborough Beach North, Rhossili
Bay, Wales (Figure 5d) and Llangenith Beach,
Wales (RUSSELLet al., 1991).
Beaches (0 > 2 and RTR > 7)
{}
Beaches with > 2 and RTR > 7 are considered
to be ultra-dissipative beaches. The term "ultradissipative" is taken from McLACHLAN(in press)
and refers to both the extreme dissipativeness of
the surf zone conditions with multiple lines of
breakers and the extreme width of the low gradient dissipative profile. Ultra-dissipative beaches are generally flat and featureless and have very
wide intertidal zones (Figure 5e and f). On ultradissipative beaches with intermediate {}values (25), surf zone conditions at high tide may be in-
mtra-Dissipative
of
+..o , o.;lio.+.o.
+-n.p.o.R.o.l"+-iu.o.
tl"'.Dhlo
,
RODPh. WIHt11-lT
..
~~
-'~
-7
-
al., 1982b) whereas on ultra-dissipative beaches
with !J >5 (e.g. Pen dine Sands; JAGO and
HARDISTY,1984: Figure 3), surf zone conditions
will be dissipative throughout the tidal cycle. Ultra-dissipative beaches are common in central
Queensland (Farnborough Beach south, Sarina
Beach, Bucasia Beach). Aspects of the dynamics
and morphology of British ultra-dissipative
beaches are described by BLACKLEYand HEATHERSHAw(1982), HAWLEY(1982), PARKER (1975)
and JAGOand HARDISTY(1984).
IMPACT
OF TEMPORAL
VARIATION
0 AND RTR
IN
The transformation of beach morphology illustrated in Figures 3 and 4 has several implications
for both understanding
and predicting beach
change as tide range increases. Firstly, temporal
and spatial change in beach morphology is a function of Hb' T and w, as well documented through
the dimensionless fall velocity {} (SHORT, 1987).
However, it is also a function of TR, and in combination with Hb' of RTR. In examining temporal
change of a' beach, T and particularly w, may be
considered as temporally constant parameters in
comparison to Hb and TR (neap to spring) which
usually experience greater change over time.
Wave height is the major variable involved in
temporal beach change, as summarised by SHORT
(1987). On micro-tidal beaches, waves drive beach
change at rates dependent on the prevailing and
equilibrium !J. Also, the response to rising waves
is faster than the response to falling waves
(WRIGHT et al., 1985). As tide range increases,
waves remain the prime contributor to temporal
beach change; however, as its energy is increasingly spread over a wider intertidal zone, the rates
of sediment transport per unit beach diminish
and beach change slows.
Tide range itself has a degree of temporal variability through the semi-diurnal inequality and
the spring to neap tidal cycle. Minor impacts of
the spring to neap tidal cycle on micro-tidal high
wave energy beach morphology have been reported by CLARKEet al. (1984) and WRIGHTet al.
(1986, 1987). A fourteen day study at Nine Mile
Beach (Yeppoon, central Queensland; Figure 5c)
revealed that the only clearly tide-induced morphological change was the migration of a swash
ridge. The ridge formed during neap tides around
high tide level and, consequently moved up the
beach as the tide range increased. Additional mornhn1nai"'Al
-_.-- UI",,1r
.. --'~
o-~- ...hAna"
0- nhQ"",~ N11'rina0 thiQ
--- t,Uln
period, such as the removal of the swash bar and
the transverse bar-rip morphology, was primarily
wave-driven. The impact of the neap-spring temporal variation in tide range is therefore secondary to waves and will not generate any substantial
change in morphology or produce a shift to an
adjacent beach type. However, the tide range does
determine the beach type and its location in Figure 4. Tide range is therefore more of a spatial
then temporal variable regarding its influence on
beach morphology.
Journal of Coutal Research, Vol. 9, No.3, 1993
Consequently, a beach type diagram can be constructed in order to locate beaches with certain
environmental conditions and suggest how beach
state may change under the influence of changing
wave (and tide) conditions. Figure 6 illustrates
such a diagram for a set wave period (T
= 8 sec)
and sediment fall velocity (w.
= 0.04 m/sec), but
with variable Hb and TR.
As discussed above, beach morphology is relatively insensitive to temporal change in tide range
(?eap to spring cycle) and, in addition, increasing
tIde range retards beach response. Consequently,
temporal beach change is primarily wave driven,
and the amount and rate of temporal change will
deaease with increasing tide range, and also with
decreasing
wave height
Beuch Model
795
wave
period
=8 see;
e
IMPORTANCE OF SWASH, SURF ZONE AND
SHOALING WAVE PROCESSES OVER A
LUNAR TIDAL CYCLE AS A
FUNCTION OF RTR
In the proposed model, three relative tide range
thresholds are proposed on the basis of the relative importance of swash, surf zone and shoaling
wave processes as illustrated in Figure 1. For RTR
<3, swash and surf zone processes dominate the
entire intertidal and the upper part of the subtidal
zone. For RTR between 3 and 7, the upper part
of the profile is dominated by swash and surf zone
processes and the lower part is dominated by
shoaling waves. For RTR > 15, only on the extreme upper part of the beach profile do swash
and surf zone processes play an important role.
This analysis was extended by running MAsSELINK's (in press) simulation model over half a
lunar tidal cycle (neap to spring) rather than over
half a tidal cycle (low to high tide) as in Figure
fan velocity
=0.04
mJs
.'
transition;
totidal /
flat f
';'8
00
c::
~
~
:g
is..
en
~
QJ
~
2
:
e ,': ~
:
~:<:';?:
'::':
0
''' '
'
,...'
~
i
- .-..
~~.~~
":
~
0
..,.,........
.
""/1
LTT
E
dissipalNe
~
..,.,...
~ tide
bwMp._J......-.
/~E-l/
/
4
non-barred
~
6
bIJ
.5
/ : uttra-dissipa~.,...
0.5
~~
1"""'."""'"
1.0
breaker
(WRIGHT et al., 1985). The
response to rising waves is also larger than that
of falling wave (WRIGHTet ai., 1985). The relative
variability of temporal beach change as a function
of wave height and tide range is indicated by the
arrows in Figure 6.
As suggested by Figure 6,beaches with large
relative tide range may be considered relatively
stable beach systems. This is supported by results
of JAGO and IiARDISTY(1984) who monitored relatively minor temporal changes in morphology on
Pendine Sands (MSR = 7.5) over a three year
monitoring period. Another indication of the morphological stsbility of beaches with large RTR
values is the often sharp break in grain size betwleen the high tide and intertidal beach of the
1<M'tide terrace beaches.
sediment
10
1.5
height
2.0
2.5
(m)
Figure 6. Location of modal beach types hased on threshold
values of breaker height and mean spring tide range for a beach
with wave period T
~
8 sec and the fall velocityw. ~ 0.04m/sec
(D.. - 0.3 mm). The boundaries between beach types, as indicated hy the dotted lines. will shift in response to changes in T
and w.. The arrows in the diagram indicate the direction and
relative, not absolute, rate of response to temporal changes in
wave height. Beach response decreases with increasing tide range
and decreasing wave height, and response to riaing waves is
faster than to falling waves (WRIGHT et al., 1985).
1. In order to generate a considerable neap to
spring variation, two tidal constituents are considered, the principal lunar (M2) and the principal solar (S2), whereby S2 is given half the amplitude of M2. Spring tide range is then given by
2 (M2 + S2) and neap tide range is 2 (M2 - S2).
Other input parameters to the simulation are H
= 1 m, T = 8 sec and w. = 0.03 m/sec. Three
different runs were performed with spring tide
ranges (and relative tide ranges) of 3, 7 and 15 m.
It should be noted that running the model with
different absolute H and TR, but with identical
relative tide range (3, 7 and 15), would produce
similar results with only swash processes slightly
increasing in importance with decreasing H and
TR.
Figure 7 shows the relative occurrence of swssh
surf zone and shoaling wave processes and th~
resulting beach profile for the three runs. It is
apparent that for RTR <3, the majority of the
intertidal zone is dominated by swash and surf
zone processes, and only seaward of neap low tide
level do shoaling waves start playing a significant
role. At mean sea level, swash and surf zone processes operate 90% of the time.
For a RTR of 7, the occqrrence of swash and
surf zone processes displays 4 maxima, decreasing
Jouma1 of Coastal Research,
Vol. 9, No.3,
1993
Conceptual
Ma.selink
796
.....,..
100
100
...,.,
..'..
~0
g
=
.~
IS,)
>
U
-5
50
0
distance (m)
-10
150
100
0
,.
100
,-...
\
.,
"""""
~II.)
KI'R = 15
~II.)
t:
0=
0
0
springhigh
- ,-- -
nea hi
~
nea low
c::
in J()\Y
.~
0>
-0
1200
-200
distance (m)
I -- ---
.--
surf
swash
zone
"... shoaling waves
I
.
. .."
d h 0 ali ng wave processes for relative tide ranges RTR of 3, 7 and 15 calculated
Figure 7. Relative occurrence of swaah, surf wne
s)
.
. agSl!11
by MAssEUNK (in press). The input
lng
uI tion model described
range = 3, 7 and
over a complete half lunar tidal cycle (neap to aprmg
fall velocity = 0.03 m/sec and tide
od = aec, ~iment
I
m,
wave
pert
paxamete ra to th em odel are wave height
t
=
k
. tal
seal.. for each beacb are different, but the vertical exaggeration is ept constan .
15 m. Note that the verti"",
'-' an d bortZOn
~
797
cess .is not necessarily an absolute measure of the
relative importance of this process. Swash and
surf 'ZOne processes are more energetic than shoaling wave processes and may be more important
even when they operate for a shorter time period.
Results of WRIGHT et at. (1982b), however, suggestthat
the relative time of occurrence of swash,
surf and shoaling waves may serve as a surrogate
...'........
RTR = 7
KI'R = 3
0
c::
IS,)
t:
:1
U
f,J
0
Beach Model
and Short
.
~
,
,
in amplitude
and cross-shore spacing in. seaw~d
direction. These maxima are associated With spr~ng
high tide, neap high tide, neap low tide and sprmg
low tide level, respectively.
At mean sea level, less
than 50 % of the time do swash and surf processes
operate. On Nine Mile Beach,which
is characterised by a RTR of 5--6, bar morphology
was
qbserved just below neap low tide level. Bar morphology may develop at this location because surf
zone processes concentrate
at this location fo~ a
sufficient amount of time, illustrated by the third
maximum in surf zone occurrence in Figure 7. T~e
fourth maximum
around spring low tide level IS
too small' and surf zone processes do not have
enough time at this location too fo.rm bar~. At ~he
two upper maxima, associated with sprmg high
and neap high tide level, surf zone processes.can
potentially
form bars. However, at these locations
swash processes play an importJJnt role and will
tend to plane off any surf zone process-induced
irregularity. Hence, no bars will form at th~se
locations although swash bars may be found instead (Nine Mile Beach, Figure 5c). It may be
suggested, on the basis of the simulation model,
that in order to develop bar morphology, surf zone
processes should dominate at least 25% of the
time over a lunar tidal cycle.
For a RTR of 15, swash and surf zone occurrence display similar maxima as was observed for
a RTR of 7 (Figure 7). However, only between
neap high tide and spring high tide do swash and
surf dominate over shoaling waves. At mean sea
level, less than 20% of the time over a half lunar
tidal cycle do swash and surf zone processes operate.
The relative time of occurrence of a certain pro-
short period waves, large tide ranges and fine sediments. This encompasses a very wide range of
environments and, therefore, not surprisingly,
ridge and runnels have been observed on low tide
terrace beaches (e.g. Druridge Bay, KING, 1972;
Dundrum Beach, ORFORD,1985), on beaches with
bar/rip-morphology at low tide level (e.g. Nine
Mile Beach, this study; Oregon beaches, Fox and
DAVIS,1978) and on ultra-dissipative beaches (e.g.
in assessing
its importJJnce. WRIGHT et al. (1982b)
Blackpool Beach, KINGand WILLIAMS,1949; West
have calculated the time-averaged surf zone energy dissipation rate relative to the total dissiLancashire, PARKER,1975; WRIGHT, 1984), some
pation rate and its distribution over the intert.iQ~
:::f"'hid. ai'e iocateci in Figure 3. Therefore, rather
prome of Cable Beach over a hiill' lunar tidal cycle.
than identifying ridge and runnel beaches as a
They only considered energy dissipation rate by
separate group, we acknowledge ridge and runnel
topography as being an additional morphological
bed friction, or equivalently the rate of doing work,
which is probably fundamental to molding the
feature which may be present on beaches with a
relative tide range larger than 3.
beach morphology
(WRIGHT et al., 1..982b). For H
Another additional morphological feature which
= 1-5 m, MSR =9.5 m and T = 11 sec (RTR ""
6), theyfound that at mean sea level 45-50% of
needs to be addressed is that of multiple bar systhe total work being done by waves is by surf zone
tems. SHORTand AAGAARD
(in press) have shown
that on wave-dominated micro-tidal beaches (RTR
waves- According to the siqlUlation model for a
<3), bar number increases with decreasing beach
RTR of 7, around 50% of the time over a half
gradient and wave period. In Figure 4, multi-bar
lunar tidal cycle do swash and surf zone processes
operate at mean sea level, in agreement with
beaches should occur as multi-bar versions of the
barred, the barred dissipative and the low tide
WRIGHT et at. (1982b).
bar/rip type. In fact, analysis of aerial photoDISCUSSION
graphs has revealed that Nine Mile Beach (FigA conceptual beach model is presented which
ures 4 and 5c) occasionally develops a double bar
system.
relates the overall beach morphology (beach state)
to hydrodynamic
and sedimentological parameSince wave energy level is only considered in a
tel&.. The model is a continuation of the work of
relative way, it is tempting to scale the model
WRIGHT and SHORT (1984) and SHORT (1991).
down and apply it to very low wave energy beach
Several elements from these studies are incorenvironments such as estuarine and bay beaches.
po;ra.ted in the model presented here. In general,
Generally these environments have large relatively tide ranges (RTR >7) and variable dimenbeaches with RTR <3 are found in micro-tidal
environments
and may be classified according to
sionless fall velocities. Using the model in Figure
4, these beaches may be described as low tide
WRIGHT and SHORT (1984). Beaches with RTR
between 7 and 15 largely overlap with Group 1 of
terrace or ultra-dissipative beaches (depending
mainly on the sediment size) which agrees with
SHORT (1991), which he sUIIimarized as macrotidal beaches with a planar, concave upward beach
what is observed in the field (NORDSTROM,1992).
profile. The transition to tidal flats (Group 3 of
However, the absolute wave energy level is also
SHORT, 1991) is indicated in the present model
of importance due to the existence of wave energy
by the beaches with RTR larger than 15. Beaches
thresholds. For example, the wave height may drop
with RTR between 3 and 7 are generally high to
below some critical level below which the response
time becomes infinite; i.e., no change occurs. Also,
moderate energy macro-tidal (TR >3 m) beaches
which were not included in any of the previous
a minimum wave energy level is required for the
excitation of infragravity edge waves (GUZAand
models.
DAVIS,1974), which are strongly implicated in the
Beaches with ridge and runnel systems (Group
formation of rhythmic topography (BOWEN and
2 of SHORT, 1991) have not been identified as a
INMAN, 1971; HOLMANand BOWEN, 1982; AAseparate beach type in the proposed model. According to SHORT(1991) and others (e.g. ORFORD GAARD,1988b).lfthe wave energy level is too low,
infragravity edge waves may not form and bar
and WRlGHT, 1978), ridge and runnel topography
morphology may not develop, even when the valis formed under the influence of moderate energy,
Journal of Coastal Research. Vol. 9, No.3, 1993
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--
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~-
-~-~----
;,
Cont:eptuaJ Beach Model
Masseliflk and Short
i~jS
ue.s of f) and TR/Hb predict the formation of bars.
Therefore, care should be exercised when applying the model to very low wave energy environments (Hb <0.25 m).
Also as the converging threshold lines in Figure
6 indicate, at low values of tide range and wave
height, small differences in one or the other can
theoretically result in markedly different morphological response. The morphological sensitivity to small changes in environmental parameters
has also been shown by DAVISand HAYES(1984)
for the micro-tidal low wave energy Florida coastline. More detailed observations of these systems
are required to clearly delineate both the wave
energy thresholds required to produce certain
morphodynamic systems, such as standing and
progressive edge waves and their morphological
imprint, in addition to the relative contribution
of waves and tides in these systems.
The absolute tide range is also of importance.
Recent work of TURNER (in press) suggests that
the formation of a low tide terrace is related to
the drainage capacity of the beach in relation to
the tide. Since this is a complex function of the
sediment characteristics (permeability and porosity), beach gradient, the duration of the tidal
cycle and the absolute tide range (TURNER, in
press), the RTR value of 3 separating the reflective beaches from the low tide terrace beaches is
rather arbitrary.
termediate Q (2-5), beaches with various bar configurations are produced in micro-tidal environments (RTR <3) (WRIGHT and SHORT, 1984),
while increasing tide range moves the rhythmic
surf topography down to the low tide level, producing low tide bar and rip morphology. On higher
wave dissipative beaches (Q >5), the beach contains multiple subdued bars on micro-tidal beaches. As tide range increases (RTR is 3-7), the bars
disappear and a wide non-barred dissipative beach
results. When RTR is 7-15 on both intermediate
and dissipative beaches (Q >2), wide, flat and
featureless ultra-dissipative beaches result. As the
relative tide range increases even more (RTR > 15),
it is suggested that the resulting beaches form the
transition to tidal flats, which are fully tide-dominated.
It is stressed that the model presented in this
paper is conceptual. Especially for the beaches
with large relative tide ranges (low tide terrace
beaches and ultra-dissipative
beaches), our
knowledge is quite restricted. More information
is required on these systems to improve .our understanding of their morphodynamics so that not
only a better understanding of the controlling
processes is attained, but also more rigourous
thresholds can be delineated to separate the different beach types. The conceptual model presented in this paper may provide a framework in
which this future work is carried out.
ACKNOWLEDGEMENTS
CONCLUSIONS
Natural beaches may be grouped into several
beach types on the basis of breaker height (Hb),
wave period (T), high tide sediment fall velocity
(w,) and tide range (TR). These four variables are
quantified by two dimensionless parameters: the
dimensionless fall velocity (Q = Hb/w,T) and the
relative tide range (RTR = TR/Hb)' The mean
spring tide range (MSR) is used to calculate the.
relative tide range. The value of the dimensionless
f~!l ".'~l~~ity i::dic2.~z
Wh2t~~:, :,~f!c~t~;~a, ~~tz:'-
mediate or dissipative surf zone conditions will
prevail. The relative tide range indicates the relative importance of swash, surf zone and shoaling
wave processes.
Small values of f) and low relative tide ranges
produce the classic reflective beach type. Increasing relative tide range results in the formation a
low tide terrace at the base of the beach face and
low tide rips, grading with increasing tide range
into a steep (reflective) beach face fronted by a
wide dissipative low tide terrace. In areas of in-
Journal
Many thanks to Ian Turner for assisting with
the surveying of the central Queensland beaches
and the exchange of ideas. Funds for this study
were generously provided by the Department of
Geography, University of Sydney (GM) lind the
Australian Research CQuncii (ADS).
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